This application claims priority under 35 USC §119 to Korean Patent Application No. 2006-40784, filed on May 6, 2006 in the Korean Intellectual. Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to analog baseband circuits such as used in direct conversion receivers, and more particularly, to elimination of DC offset in an analog baseband circuit with minimized transients for preserving signal integrity.
2. Background of the Invention
Direct conversion receivers, such as Wideband Code Division Multiple Access (WCDMA) receivers, continuously receive signals during communications. Such receivers usually remove a DC offset using high pass filtering such as AC coupling or DC feedback loop. However in most modulations, significant information is positioned near DC such that a −3 dB frequency of a high pass filter should be set to be as low as possible to prevent signal quality from decreasing. On the other hand, a low cutoff frequency prolongs the time for removing a rapidly and quickly changing DC offset. Accordingly, high pass filtering is not appropriate for removing a rapidly and quickly changing DC offset.
The rapidly and quickly changing DC offset may occur when a gain of an analog baseband circuit is changed in stages by a digitally controlled programmable gain amplifier (PGA). With prolonged time for removing the DC offset, a large-step transient that attenuates slowly occurs in a receiver output signal. A transient may deteriorate signal quality or saturate a back-end (e.g., a modem) of a receiver.
Methods of quickly attenuating a transient by increasing a cutoff frequency in an instant when a gain changes have been introduced by P. M. Stroet et al. [“A Zero-IF Single-Chip Transceiver for up to 22 Mb/s QPSK 802.11b Wireless LAN”, IEEE International Solid-State Circuit Conference Digest of Technical Papers, February 2001, pp. 204-205] and W. Schelembauer et al. [“Analog BaseBand Chain for a UMTS Zero-IF Receiver in a 75 GHz SiGe BiCMOS Technology”, IEEE Radio Frequency Integrated Circuits Symposium Digest of Papers, May 2002, pp. 267-270].
However in these conventional methods, since the cutoff frequency is increased at the instant when a gain changes, signal quality may be deteriorated during that short time. In particular, a bit error rate (BER) of a receiver may be increased in a multi-path fading environment in which a gain changes rapidly and quickly.